US20090086358A1 - Method and system for magnetic recording using self-organized magnetic nanoparticles - Google Patents
Method and system for magnetic recording using self-organized magnetic nanoparticles Download PDFInfo
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- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
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- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y25/00—Nanomagnetism, e.g. magnetoimpedance, anisotropic magnetoresistance, giant magnetoresistance or tunneling magnetoresistance
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- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/012—Recording on, or reproducing or erasing from, magnetic disks
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- H01F1/0036—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties showing low dimensional magnetism, i.e. spin rearrangements due to a restriction of dimensions, e.g. showing giant magnetoresistivity
- H01F1/0045—Zero dimensional, e.g. nanoparticles, soft nanoparticles for medical/biological use
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- H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
- H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
- H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
- H01F1/06—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys in the form of particles, e.g. powder
- H01F1/068—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys in the form of particles, e.g. powder having a L10 crystallographic structure, e.g. [Co,Fe][Pt,Pd] (nano)particles
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Abstract
Description
- This application is a divisional application and claims the benefit of U.S. patent application Ser. No. 11/314,846 filed Dec. 21, 2005, which is a continuation of U.S. patent application Ser. No. 10/683,383 filed Oct. 10, 2003, now U.S. Pat. No. 7,029,773 B2, which are incorporated herein by reference.
- The present invention relates to magnetic recording and, more particularly, to a method and system for magnetic recording that utilizes a magnetic recording media having self-organized magnetic nanoparticles.
- In the field of magnetic recording, areal density is an important factor driving the design of future magnetic recording systems. Increased storage capacity in magnetic recording has traditionally been addressed through improvements in the ability to store information on a particular storage disc having an increased areal density. Conventional longitudinal and proposed perpendicular recording schemes have been projected to include areal densities of about 1 Tbpsi, but will require extensive modifications to allow further growth.
- Accordingly, much attention has been directed toward either improving the various components of a conventional magnetic recording system or developing new types of magnetic recording systems. For example, self-organized magnetic arrays of nanoparticles have been produced and investigated for use as magnetic recording media for future ultra-high density magnetic recording applications. These nanoparticles may provide conceivable solutions to many proposed future recording schemes, e.g., conventional granular media, perpendicular media, thermally assisted recording media, patterned media recording schemes and probe storage systems. While much effort has been directed toward the various potential applications of the self-organized magnetic nanoparticles for use in magnetic recording media, much more effort is needed for incorporating such proposed media into an entire magnetic recording system for performing read and/or write operations.
- There is identified, therefore, a need for improved magnetic recording systems that overcome limitations, disadvantages, or shortcomings of known magnetic recording systems.
- An aspect of the present invention is to provide a method of magnetic recording that comprises depositing surfactant coated nanoparticles on a substrate, wherein the surfactant coated nanoparticles represent first bits of recorded information. The method also includes removing the surfactant coating from selected of the surfactant coated nanoparticles. The selected nanoparticles with their surfactant coating removed may then be designated to represent second bits of recorded information. The surfactant coated nanoparticles have a first saturation magnetic moment and the selected nanoparticles with the surfactant coating removed have a second saturation magnetic moment. Therefore, by selectively removing the surfactant coating from certain nanoparticles, a write operation for recording the first and second bits of information may be performed. A read operation may be carried out by detecting the different magnetic moments of the surfactant coated nanoparticles and the non-surfactant coated nanoparticles.
- Another aspect of the present invention is to provide a magnetic recording system that comprises a recording medium having a substrate with surfactant coated nanoparticles and non-surfactant coated nanoparticles. The surfactant coated nanoparticles represent first bits of recorded information and the non-surfactant coated nanoparticles represent second bits of recorded information. The magnetic recording system also comprises means for writing the first and second bits of recorded information and means for reading the first and second bits of recorded information.
- A further aspect of the present invention is to provide a method of magnetic recording that comprises depositing a layer of self-organized magnetic nanoparticles on a substrate. The method also includes altering a magnetic property magnitude of selected of the self-organized magnetic nanoparticles and designating bits of recorded information according to the magnetic property magnitude of either the self-organized magnetic nanoparticles or the magnetic property magnitude of the selected self-organized magnetic nanoparticles that were altered.
- These and other aspects of the present invention will be more apparent from the following description.
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FIG. 1 is a schematic illustration of a magnetic recording system constructed in accordance with the invention. -
FIG. 2 is an enlarged view of a nanoparticle that may be used in accordance with the invention. -
FIG. 3 is a schematic illustration of a self-organized magnetic array medium. -
FIG. 4( a) is a transmission electron microscope (TEM) image of an FePt nanoparticle sample. -
FIG. 4( b) is a graphical illustration of particle diameter distribution for the FePt nanoparticles illustrated inFIG. 4( a). -
FIG. 5( a) is a graphical illustration of weight loss rate for a surfactant coating material. -
FIG. 5( b) is a graphical illustration of weight loss rate for a FePt nanoparticle solution deposited onto a silicon wafer and measured by a thermogravimetric analyzer (TGA). -
FIG. 6 is a graphical illustration of magnetization data for various annealing conditions. -
FIG. 7 is a graphical illustration of a magnetization curve for use in estimating particle size. - The invention relates to magnetic recording and, more particularly, to a method and system for magnetic recording that utilizes a magnetic recording media having self-organized magnetic nanoparticles. The invention includes altering a magnetic property magnitude, such as saturation magnetic moment or other magnetic properties of the nanoparticles, of the self-organized magnetic nanoparticles and distinguishing between the unaltered and altered nanoparticles for purposes of recording bits of information.
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FIG. 1 illustrates an embodiment of amagnetic recording system 10 constructed in accordance with the invention. Specifically, therecording system 10 includes arecording head 12 and a recording medium generally designated byreference number 14. Therecording medium 14, which may travel in the direction of arrow A relative to therecording head 12, includes asubstrate 16 and a layer of nanoparticles, generally designated byreference number 18, deposited on thesubstrate 16. Thesubstrate 16 may be formed of any suitable material such as, for example, amorphous or nano-crystalline (ceramic) glass, NiP coated Al, Si, SiO2, and thermally oxidized Si. Thesubstrate 16 may have a thickness T1 in the range of about 0.1 mm to about 2.0 mm. - The layer of
nanoparticles 18 may be deposited on thesubstrate 16 using, for example, dip-coating where thesubstrate 16 is submerged in a liquid containing thenanoparticles 18 and subsequently controllably extracted. Alternatively, a spin-coating process may be used where a nanoparticles-containing fluid is applied to the surface of thesubstrate 16 followed by a controlled spinning of thesubstrate 16 to remove excess materials. -
FIG. 2 illustrates an example of ananoparticle 18 in more detail. Specifically, thenanoparticle 18 includes ananoparticle core 20 that may be formed of, for example, FePt, CoPd, Co, CoFe, Fe or Ni. Thecore 20 may have a diameter D1 in the range of about 2 nm to about 8 nm. In addition, thenanoparticle 18 may have ashell 22 that at least partially surrounds thecore 20. Theshell 22, as will be explained in more detail herein, provides a surfactant coating around thecore 20. Theshell 22 may be formed of, for example, oleic-acid molecules or thiols. Accordingly, thecore 20 andshell 22 combine to form ananoparticle 18 having a total diameter D2 in the range of about 4 nm to about 12 nm. - Referring to
FIG. 1 , therecording head 12 includes a write element, generally designated byreference number 24, and a read element, generally designated byreference number 26. Thewrite element 24, in one embodiment, may include an electron emitting device such as afield emission tip 28 which can be used to create a flow of electrons that form an electron current, as designated byreference number 30, from therecording head 12 toward therecording medium 14 by the application of a voltage. Thefield emission tip 28 controls theelectron current 30, i.e., the write current, by the tip voltage. - The writing process utilizing the
write element 24 generally involves the local removal of the surfactant coating orshell 22 fromindividual nanoparticles 18. Eachnanoparticle 18, as deposited on thesubstrate 16, has a first saturation magnetic moment. By turning on theelectron current 30, theshell 22 surrounding thecore 20 of anindividual nanoparticle 18 will dissolve locally leaving behind only thecore portion 20. This results in a non-surfactant coatednanoparticle 20 that has a second saturation magnetic moment that is greater than the first saturation magnetic moment of theoriginal nanoparticle 18. The details regarding local removal of the surfactant coating orshell 22 of thenanoparticle 18 in order to alter the saturation magnetic moment thereof will be described in more detail herein. - Accordingly, it will be appreciated that the
original nanoparticles 18 may each be designated to represent a first bit of recorded information in themagnetic recording system 10. In addition, the non-surfactantcoated nanoparticles 20 having a second saturation magnetic moment may be designated to represent second bits of recorded information for themagnetic recording system 10. - In other embodiments of the invention, the
write element 24 may include, for example, a localized heat source, such as a focused laser spot or a near field optical spot for locally removing the surfactant coating or shell 22 from selectednanoparticles 18. - Referring to
FIG. 1 , theread element 26 of therecording head 12 is provided for performing a read operation on the surfactant coatednanoparticles 18 and the non-surfactantcoated nanoparticles 20. The readelement 26 is structured and arranged, in one embodiment, for detecting and distinguishing between the surfactant coatednanoparticles 18 and the non-surfactantcoated nanoparticles 20. More specifically, theread element 26 is structured and arranged for detecting the saturation magnetic moment of the surfactant coatednanoparticles 18 and the saturation magnetic moment of the non-surfactantcoated nanoparticles 20 and distinguishing therebetween. As described, the surfactant coatednanoparticles 18 may be designated to represent first bits of recorded information and the non-surfactantcoated nanoparticles 20 may be designated to represent second bits of recorded information. - Still referring to
FIG. 1 , theread element 26 may include acoil 32 that produces a magnetic field H and amagnetoresistive read element 34 positioned relative to thecoil 32. The magnetic field H is exposed to a particular nanoparticle, e.g., either a surfactant coatednanoparticle 18 or a non-surfactantcoated nanoparticle 20, and the magnetic field H will polarize the magnetic moment of the subject nanoparticle to which it is exposed. The degree of polarization of the nanoparticles depends on the total moment of the particles, i.e., whether it is one of the surfactant coatednanoparticles 18 having a saturation magnetic moment that is less than a saturation magnetic moment of one of the non-surfactantcoated nanoparticles 20. Thus, the difference in polarization between the surfactant coatednanoparticles 18 and the non-surfactantcoated nanoparticles 20 can then be detected using the MR readelement 34. - In another embodiment, the
read element 26 rather than utilizing the described magnetic polarizing field, may employ a time varying field using a coil or microscopic electromagnet. The alternating field will polarize the nanoparticles, such asnanoparticles element 26. The information can be retrieved by demodulation of the signal in the readelement 26. It will be appreciated that other configurations may be employed to provide a read operation by detecting the saturation magnetic moment of the individual nanoparticles or by detecting other variable magnetic properties of the nanoparticles as well. -
FIG. 3 illustrates an additional embodiment of arecording medium 114 constructed in accordance with the invention. In contrast to therecording medium 14 which includes a uniform or continuous layer ofnanoparticles 18 deposited thereon in, for example, a grid type pattern or arrangement, therecording medium 114 includesarrays 117 each comprisingmultiple nanoparticles 118. Thearrays 117 ofnanoparticles 118 are deposited on a substrate 116 of therecording medium 114. Thearrays 117 ofnanoparticles 118 may be spatially organized incircumferential tracks 119 of therecording medium 114 by using, for example, macroscopic topographic features on a length scale smaller than the correlation length of the self-organization process during the deposition phase of the preparation. For example, this length scale can be at least several microns long. These macroscopic features can easily be fabricated to follow the desired circumferential or other pattern. - Further details regarding the theory of operation of the
recording head 12 illustrated inFIG. 1 is as follows. For an ensemble of non-interacting paramagnetic nano-particles, the magnetization density M as a function of the applied magnetic field H is given by the Langevin function: -
M=Nm[coth(μ0 mH/k B T)−k B T/μ 0 mH] - where μ0=4η10−7 is the permeability of vacuum, N is the number of nano-particles per volume, and m is the net moment of the nano-particle. For the case of (μ0mH/kBT)=1, the relative moment (normalized to the full moment of the nano-particle) can be written as:
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|M/Nm|=μ 0 mH/3k B T - or as the initial magnetic susceptibility χ
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χ=M/H=Nμ 0 m 2/3k B T - Note that χ has a quadratic dependence on the moment m of the individual nano-particles. Therefore, a change Ms will correspond to a larger change in the observed read-back signal, which is proportional to χH.
- For a typical nano-particle with a diameter d=4 nm and a saturation magnetization of Ms=800 emu/cm3, the moment mmod=(π/6)d3Ms=26.8 10−18 emu. For the same nano-particles the measured value was Ms=200 emu/cm3 in the as-prepared state, which would correspond to mini=6.7 10−18 emu for a 4-nm-diameter nano-particle.
- The (maximum) applied magnetic field is chosen such that μ0mH/kBT=1 for the modified nano-particles. In the present example and using T=300 K, this results in a relatively easy to achieve field of H=123 kA/m (=1.54 kOe). The moment that will be sensed by the read-back device is Mmod=0.313 mmod=8.39 10−18 emu (per nano-particle). For the non-modified nano-particles, this moment is Mini=0.083 mini=0.56 10−18 emu (per nano-particle). The contrast ratio is (Mmod−Mini)/
M ini 100%=1400%. - Note that present read-back devices are designed to be sensitive to moments of the order of Mpresent=1.55 10−15 emu (taking 10% of the volume of a bit-cell with an area A=2,581 nm2 [corresponding to a bit-density of 250 Gbit/in2], a thickness t=10 nm, and an Ms=600 emu/cm3), which is 185 times larger than for the signal of a single modified nano-particle. Assuming that the full positive to negative swing of a present medium corresponds to a signal-amplitude of 5 mV, the paramagnetic signal would correspond to a signal-amplitude of 13 μV (or 9 μVrms for ac detection.
- The following example explains in detail the concept of removing a surfactant coating from an as deposited nanoparticle structure and, particularly for self-assembled, monodisperse L1 0 FePt nanoparticles for forming a magnetic data storage media, such as
magnetic recording medium 14. The chemically ordered L1 0 phase of the FePt system is of particular interest, because of its high bulk magnetocrystalline anisotropy energy density (Ku˜6.6×107 ergs/cm3) at the equiatomic composition that should allow the use of smaller, thermally stable magnetic grains than is generally used in current recording media. - In one embodiment of the present invention, there is provided a magnetic recording system that alters and detects saturation magnetic moment for recording information. Thus, this example concentrates on the saturation magnetization, MS. In Fe50Pt50 thin films, MS is about 1125 emu/cm3, close to the bulk value MS=1140 emu/cm3. Techniques for these measurements are well established. In chemically synthesized, surfactant-coated FePt nanoparticle systems, however, quantitative MS measurements are complicated by the difficulty of determining weights or volumes without introducing large errors. We provide a systematic study of MS of FePt nanoparticle systems at different annealing temperatures and constant annealing time (30 minutes). The amount of the material is characterized by weighing the FePt nanoparticles (˜1 mg with 0.1 μg accuracy) after the surfactant coating decomposes at temperatures T≧400° C.
- The present assemblies of FePt nanoparticles are synthesized using techniques that are known. Thermally oxidized silicon coupons are used as substrates.
FIG. 4( a) shows a plan-view transmission electron microscope (TEM) image of a representative assembly. A median particle diameter of 3.1 nm with 0.15 nm standard deviation (σ=0.05) is formed (FIG. 4( b)). The as-prepared samples are then annealed in a Rapid Thermal Annealing furnace (RTA) under argon environment at various temperatures Tanneal=250° C.˜650° C. to induce L1 0 chemical ordering and to generally explore changes in magnetic and micro-structural properties. The inert argon environment during annealing prevents external oxidation or reduction of the FePt nanoparticles. The annealing time is fixed at 30 minutes for all the samples. Magnetic measurements are performed using a Superconducting Quantum Interference Device magnetometer (SQUID) with 50 kOe field range. All measurements reported here are carried out at low temperatures (T≦30 K) to avoid complications due to size dependent superparamagnetic effects. - In order to characterize the total mass of the FePt nanoparticles, a Thermogravimetric Analyzer (TGA) is used to measure the weight and to monitor the weight change of the FePt nanoparticles/surfactant systems as function of annealing temperature. The TGA used is a TA Instrument (TGA-2950) with 0.1 μg weight resolution. Since the range of the FePt solution weight used for magnetization measurements is from 0.5 mg˜3 mg, the error bar due to instrument resolution is negligible. The sample is placed on a Pt sample pan in the TGA chamber and is then heated to temperatures up to 650° C. at controlled scan rate up to 10° C./min. Nitrogen gas is flushed through the TGA chamber to remove oxygen. The weight change is monitored in-situ during heating.
- The example first investigates a surfactant-only sample. A few drops (about 16 mg) of the oleyl acid/oleylamine 50:50 surfactant mixture are put directly into the sample pan. The measured weight starts dropping at about 150° C. indicating the onset of surfactant decomposition. A modest weight reduction in the range 150 to 300° C. is followed by a more dramatic drop above 300° C., reaching zero at about 400° C. (shown in
FIG. 5( a)). The weight-loss-rate, also shown inFIG. 5( a), shows a small peak at about 200° C., followed by a distinct peak at about 350° C. The weight loss is, besides surfactant decomposition, also believed to be at least partially due to evaporation. -
FIG. 5( b) shows the TGA weight-loss observed for a FePt nanoparticle film deposited on a silicon wafer. The total initial weight of the solution is about 0.6 mg. The temperature trace is very similar to that of the pure surfactant with major weight-loss occurring near 350° C. and a weak shoulder observable near 250° C. Both features can be attributed to the decomposition (or evaporation) of the surfactant. After heating to T≧400° C., the weight has stabilized. This suggests that the weight left behind after annealing at Tanneal≧400° C., which is about 50% of the initial weight, is attributable to FePt nanoparticles. The example also studied bare silicon substrates and found weak weight-loss of less than ˜0.4% of the silicon weight, which is attributed to evaporation of moisture (water). This weak weight-loss from the silicon substrate is taken into account in quantifying the weight of FePt, assuming that it behaves in a consistent manner from sample to sample. The maximum possible error resulting from this assumption is about ±5%. -
FIG. 6 shows a series of low temperature (T=5K) SQUID magnetization data (half hysteresis curves) for various annealing conditions. In order to convert from moment per unit mass (emu/g) to moment per unit volume (emu/cm3), the example starts with a FePt bulk density of 15 g/cm3, which is the average of Fe and Pt mass densities. Because of the lower atomic weight of Carbon, the total mass density (g/cm3) is only reduced by an estimated 6-8% to 13.8-14.1 g/cm3. Another uncertainty for mass density arises from the possibility that the mass density within the surface region of the nanoparticles is different from that in the interior. Assume, e.g. a 4% surface lattice constant change and take into account that about 30% of Fe and Pt atoms in a 3.1 nm diameter FePt nanoparticle reside within the first surface atomic layer, then the total particle volume change associated with such a possible surface effect is only of the order of 3-4%. This lies well within the error bar of the present measurement. For simplicity we assume a mass density of 14 g/cm3 in the following. - The coercivity HC increases from less than 200 Oe for the as-prepared sample to 22 kOe for the sample annealed at Tanneal=650° C., a sharp onset of coercivity occurring near 350° C. For Tanneal≧450° C., the magnetization at 50 kOe is about 850 emu/cm3, 25% lower than the MS of bulk FePt. This discrepancy could partly be due to the fact that at 50 kOe magnetic field, the sample is not yet fully saturated. However, for the sample annealed at Tanneal≦450° C., the magnetization is significantly reduced. The as-prepared sample has a magnetization of only about 210 emu/cm3, 19% of the bulk value for FePt. The critical annealing temperature to cause a significant increase of the saturation magnetization is about 450° C., which is consistent with the surfactant decomposition temperature. The annealing temperature dependence of the magnetization, along with the weight reduction for these RTA annealed samples is shown in the inset to
FIG. 6 . A clear correlation between the magnetization and the weight loss of the sample (decomposition of the surfactant) exists. It is noted that differences, e.g. in the onset temperature for the enhanced MS (≈450° C.) and the weight-loss peak (≈400° C.) may be related to differences in the annealing conditions in RTA and TGA. - The present findings are interpreted as due to the presence of strong interactions between the surfactant and the nanoparticle surface in the as-prepared state. Preliminary density functional calculations indicate that these surface bonding interactions take place predominately at surface iron sites, which is the primary contributor to the magnetization in FePt. The mechanism of bonding to the nanoparticle surface depends on many parameters; however, surface geometry and electronic structure are perhaps the most important factors which determine the location and strength of the surface bond. Metal surfaces containing available d-bands are known to interact with small molecules having accessible π* states through a Blyholder type interaction. For example, this interaction in carbonyl-based molecules weakens the C—O bond through charge donation from the metal d-band into this unoccupied anti-bonding state, thereby weakening the bond. Preliminary spin polarized density functional theory results from cluster models of the FePt(111) surface, which include a small molecule model of the oleylamine surfactant, show that charge is transferred to the surface Fe site from the model oleylamine, thereby lowering the atomic magnetization by about 60% at the Fe site, which is consistent with the previously described bonding mechanism and the observed decrease in magnetization of the nano-particles described above.
- On increasing the annealing temperature to 400° C., the surfactant starts to decompose and leaves the particle surface, breaking the bonding between the surfactant and the particles. Without the dead layer from the surfactant-particle interaction, the nanoparticle system recovers its full magnetization.
- It is well established that the “magnetic” particle size can be estimated by fitting to the Langevin function in the superparamagnetic regime. The Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetization curves show a blocking temperature TB=12K for as-prepared FePt samples in a magnetic field of 200 Oe. In order to estimate the particle size from the Langevin function, the magnetic field dependence of the magnetization curve is measured at T>TB. In this case, T=30K is used (shown in
FIG. 7 ). The magnetic moment per particle is then estimated by: -
- where χi is the initial susceptibility of the magnetization curve (χi=6.5×10−6 emu/Oe for this particular case) and mS is the saturation magnetic moment (in units of emu). In order to extrapolate mS, the magnetic moment m is plotted against 1/H. For superparamagnetic materials, the relationship between m and 1/H for large field should be linear with m=mS at 1/H→0 and m=0 at 1/H=1/H0. The inset to
FIG. 7 shows the m vs. 1/H plot for this particular sample for H>20 kOe. The linear extrapolation gives rise to mS=0.0264 emu and H0=4842 Oe. Using Equation (1), we obtain the MSV=172μB/particle for an as-prepared FePt sample. Consequently, the diameter of the “magnetic” particle size can be calculated to be Dmag=1.7 nm using MS=850 emu/cm3. For the as-prepared sample with a saturation magnetization of 210 emu/cm3, the ratio of the magnetic volume and physical volume for the particle Vmag/Vparticle=MS,bulk/MS,particle=25%. The particle diameter are therefore Dmag/Dparticle=(0.25)1/3=0.63, resulting in a physical particle diameter Dparticle=2.7 nm and a dead layer thickness of 0.5 nm. In conclusion of this example, it is observed that a 75% magnetization reduction for as-prepared FePt nanoparticle samples in comparison with the samples annealed at high temperatures. The strong correlation between magnetization recovery temperature (≧400° C.) and surfactant decomposition temperature (350° C.˜400° C.) suggests that the chemical interaction between Fe and surfactant at the particle is responsible for this dramatic magnetization reduction. A Langevin function analysis for as-prepared FePt particles suggests a core-shell structure of the 2.7 nm diameter FePt particles with a 0.5 nm non-magnetic shell. - Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
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